Yeast infections start when Candida, a fungus that naturally lives in your vagina in small amounts, shifts from a harmless state to an overgrown, invasive one. This shift happens when something disrupts the vaginal environment, whether that’s antibiotics, hormonal changes, or excess moisture. Up to 75% of women will experience at least one yeast infection in their lifetime, making it one of the most common vaginal conditions.
Candida Already Lives in Your Body
The fungus behind most yeast infections, Candida albicans, is a normal part of your vaginal microbiome. In its harmless form, it exists as small round yeast cells that coexist with the bacteria that keep your vagina healthy, particularly lactobacilli. These bacteria produce lactic acid, maintaining a vaginal pH between 3.8 and 4.5, which is acidic enough to keep Candida’s numbers in check.
The infection doesn’t come from “catching” something new. It starts when conditions inside the vagina change enough that Candida can grow unchecked. When the environment shifts, the fungus responds by transforming from its round yeast form into long, branching filaments called hyphae. These filaments can penetrate the vaginal lining, triggering the inflammation, itching, and discharge that define a yeast infection. Candida is remarkably adaptable: it can sense and respond to changes in nutrient availability, pH, and other chemical signals in its environment, using these cues to switch into its more aggressive form.
Antibiotics Are a Leading Trigger
Taking antibiotics is one of the most well-documented ways a yeast infection gets started. Antibiotics kill bacteria, but they don’t distinguish between harmful bacteria and the protective lactobacilli in your vagina. When those beneficial bacteria are wiped out, the acidic environment they maintain weakens, and Candida has room to multiply.
Research published in the New England Journal of Medicine found that women who developed yeast infections were significantly more likely to have used antibiotics in the preceding month. About 19% of women with Candida infections had recently taken antibiotics, compared to roughly 12% of women without infections, translating to a 75% higher likelihood. The risk also increases the longer you take antibiotics: duration of use was linearly related to the chance of developing an infection. This means a ten-day course of antibiotics carries more risk than a three-day one, regardless of the type.
How Hormones Fuel Candida Growth
Estrogen plays a direct role in how Candida behaves. Higher estrogen levels encourage the fungus to form those invasive filaments, which is why yeast infections are far more common during pregnancy, in the second half of the menstrual cycle, and in women using estrogen-containing birth control or hormone replacement therapy.
Lab research shows that when Candida cells are exposed to estrogen, they increase filament formation. Estrogen also appears to influence the production of ergosterol, a key component of Candida’s cell membrane (similar to what cholesterol does in human cells). Changes in ergosterol levels help the fungus build stronger cell walls during its invasive phase. This is why pregnancy is such a strong risk factor: estrogen levels climb dramatically, creating a sustained environment where Candida is more likely to shift into its disease-causing form.
The flip side is also true. Before puberty and after menopause, when estrogen levels are naturally low, yeast infections become much less common. A rise in vaginal pH above 4.5, which naturally occurs just before your period and after menopause, can also alter the microbial balance, though in those cases bacterial infections tend to be more common than yeast.
Moisture and Clothing Choices
Candida thrives in warm, moist environments. Anything that traps heat and moisture against the vulva gives the fungus a better chance to proliferate. Synthetic underwear fabrics are a known contributor because they don’t breathe the way cotton does. The synthetic fibers and dyes used in them can alter vaginal conditions, raising pH levels above 4.5 and creating a less hospitable environment for the protective lactobacilli that normally keep Candida in check.
Athletes are particularly prone to fungal infections in the groin area because of the combination of sweat, heat, and tight-fitting clothing during exercise. Going without underwear when possible, or switching to cotton, allows air to circulate and lets natural secretions dry, reducing the moist conditions that feed fungal growth. Sitting in a wet swimsuit or sweaty workout clothes for extended periods creates a similar effect.
Other Common Triggers
A weakened immune system makes yeast infections more likely because your body is less able to keep Candida’s growth in check. This includes people managing diabetes (especially when blood sugar is poorly controlled, since Candida feeds on sugar), those on immunosuppressive medications, and people living with HIV. Uncontrolled diabetes is one of the strongest risk factors for recurrent infections.
Douching and scented vaginal products can also set the stage. These products disrupt the natural bacterial balance in much the same way antibiotics do, stripping away protective bacteria and shifting the pH upward. Sexual activity doesn’t cause yeast infections directly (they’re not sexually transmitted), but it can introduce changes to the vaginal environment that make an overgrowth more likely in some women.
When One Infection Becomes a Pattern
Most yeast infections are isolated events, but for a smaller group of women, they keep coming back. The CDC defines recurrent vulvovaginal candidiasis as three or more symptomatic episodes within a single year, a pattern that affects fewer than 5% of women. Despite being relatively uncommon, recurrent infections carry a substantial personal and economic burden.
Recurrence often happens because the underlying trigger hasn’t been addressed. If you’re on long-term antibiotics, have unmanaged blood sugar, or are taking estrogen-based hormones, the conditions that allowed the first infection keep repeating. In some cases, the Candida strain involved may be one that’s harder to clear with standard antifungal treatments, or the immune response in the vaginal tissue may be producing excessive inflammation that damages the protective bacterial community each time, creating a cycle.
Recognizing the pattern matters because recurrent infections are typically managed differently from a one-time episode, often with a longer course of treatment followed by a maintenance phase to prevent the next overgrowth from taking hold.